Wheel loader and method for controlling a wheel loader

ABSTRACT

A tractive force control part of a wheel loader is configured to perform a tractive force control to reduce a maximum tractive force. The tractive force control part is further configured to reduce the maximum tractive force when determination conditions including, while a working implement is performing excavation operation, that the working implement is in a raising hydraulic stall condition and that a drive circuit pressure is greater than or equal to a predetermined hydraulic pressure threshold are satisfied during the tractive force control.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 13/700,412. This application claims priority toJapanese Patent Application No. 2012-078942 filed on Mar. 30, 2012. Theentire disclosures of U.S. patent application Ser. No. 13/700,412 andJapanese Patent Application No. 2012-078942 are hereby incorporatedherein by reference.

TECHNICAL FIELD

The present invention is related to a wheel loader and a method forcontrolling a wheel loader.

BACKGROUND ART

Among wheel loaders there are wheel loaders mounted with what is calledHST (Hydro Static Transmission). In an HST type wheel loader an enginedrives a hydraulic pump and a traveling hydraulic motor is driven byhydraulic fluid discharged from a hydraulic pump. This is how the wheelloader is made to travel. In an HST type wheel loader speed and tractiveforce can be controlled by controlling engine rotation speed,displacement of the hydraulic pump and displacement of the hydraulicmotor for driving the wheel loader. (Japanese Unexamined PatentPublication No. 2008-144942).

The operator of such a wheel loader can implement tractive force controlselectively. Tractive force may for example, limit displacement of atraveling hydraulic motor to an upper limit displacement less thanmaximum displacement, thereby reducing maximum tractive force. Theoperator may choose to implement tractive force control if slip or stallphenomena occur due to excessive tractive force. Doing so reducesmaximum tractive force suppressing the occurrence of phenomena such asslip or stall and the like

SUMMARY

Some types of wheel loader are constructed such that the operator canselect the level of maximum tractive force with tractive force control.The operator can select the level of maximum tractive force controlusing tractive force control in advance. Once the operator operates aswitch that implements tractive force control, maximum tractive force islimited to the selected level, thereby enabling the operator to selectthe appropriate level of tractive force in relation to for example, thecondition of a road surface.

The level of tractive force required is not constant however and mayvary with work conditions. Therefore it is not easy for an operator toselect the optimum level of maximum tractive force in advance to preventthe occurrence of phenomena such as stalling or slipping and the like.Accordingly, in the case of a conventional wheel loader, the operator isforced to continually reselect the level of maximum tractive forceduring excavation work in response to changing work conditions.

For example, even when maximum tractive force is reduced to apredetermined level using tractive force control as described above,during excavation a working implement can go into a condition of raisinghydraulic stall. In a raising hydraulic stall condition, despiteperforming the operation to raise the boom in order to raise the bucket(hereinafter referred to as “boom raise operation”), the boom does notrise. The condition of raising hydraulic stall often occurs during whatis called the “scooping in operation” which is the operation of scoopingthe material being worked such as earth and sand and the like into thebucket. In a scooping in operation the bucket is raised while pushingthe bucket into the earth and sand by advancing the vehicle forward. Atthis time a counterforce is exerted against the bucket opposing thetractive force advancing the vehicle. If the load operating against thebucket due to this counterforce becomes excessive, raising the bucketbecomes difficult. Moreover, if the load becomes greater than the driveforce of the lift cylinder that raises the bucket, it may not bepossible to raise the bucket so that the working implement enters theraising hydraulic stall condition as above. When this happens, theoperator must perform an operation to reduce the level of maximumtractive force, then when the working implement recovers from theraising hydraulic stall condition, the operator must perform anotheroperation to return the maximum tractive force to the original level.Having to perform these operations is troublesome and reduces wheelloader operability.

An object of the present invention is to provide a wheel loader and amethod for controlling a wheel loader that enables recovery from theraising hydraulic stall condition affecting a working implement duringexcavation and provides improved operability.

A wheel loader according to one aspect includes a working implement, anengine, a first hydraulic pump, a traveling hydraulic motor, a secondhydraulic pump, a working implement operating member, a drive circuitpressure detection part, a raising hydraulic stall determination part, adrive circuit pressure determination part, and a tractive force controlpart. The working implement includes a boom, a bucket and a liftcylinder that raises the bucket by moving the boom. The first hydraulicpump is driven by the engine. The traveling hydraulic motor is driven byhydraulic fluid discharged from the first hydraulic pump. The secondhydraulic pump is driven by the engine to discharge hydraulic fluid thatdrives the lift cylinder. The working implement operating member isconfigured to operate the working implement. The drive circuit pressuredetection part is configured to detect a drive circuit pressure that isa pressure of the hydraulic fluid for driving the traveling hydraulicmotor. The raising hydraulic stall determination part is configured todetermine whether or not the working implement is in a raising hydraulicstall condition, in which the bucket does not rise regardless ofoperation of the working implement operating member. The drive circuitpressure determination part is configured to determine whether or notthe drive circuit pressure is greater than or equal to a predeterminedhydraulic pressure threshold. The tractive force control part isconfigured to perform a tractive force control to reduce a maximumtractive force. The tractive force control part is further configured toreduce the maximum tractive force when determination conditionsincluding, while the working implement is performing excavationoperation, that the working implement is in the raising hydraulic stallcondition and that the drive circuit pressure is greater than or equalto the predetermined hydraulic pressure threshold are satisfied duringthe tractive force control.

According another aspect, a method is provided for controlling a wheelloader including an engine and a working implement including a boom, abucket and a lift cylinder that raises the bucket by moving the boom.The method includes: determining whether or not a raising hydraulicstall condition is occurring, in which the bucket does not rise eventhough hydraulic fluid discharged from a second hydraulic pump, which isdriven by the engine, is supplied to the lift cylinder; determiningwhether or not a drive circuit pressure of a drive circuit, throughwhich hydraulic fluid discharged from a first hydraulic pump driven bythe engine is supplied to a traveling hydraulic motor, is greater thanor equal to a predetermined hydraulic pressure threshold; and, during atractive force control for reducing a maximum tractive force isperformed, further reducing a maximum tractive force when determinationconditions including, while the working implement is performingexcavation operation, that the raising hydraulic stall condition isoccurring and that the drive circuit pressure is greater than or equalto the predetermined hydraulic pressure threshold are satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a wheel loader according to an embodiment ofthe present invention;

FIG. 2 is a block diagram of the configuration of a hydraulic drivingmechanism mounted on the wheel loader;

FIG. 3 shows the engine output torque line;

FIG. 4 shows an example of the characteristics of pump displacement todrive circuit pressure;

FIG. 5 shows an example of the characteristics of motor displacement todrive circuit pressure;

FIG. 6 shows an example of a graph of vehicle speed of the wheel loaderand tractive force;

FIG. 7 shows an example of tractive force ratio information;

FIG. 8 is a block diagram showing the configuration of a vehiclecontroller;

FIG. 9 is a flowchart showing the determination process forautomatically reducing maximum tractive force during tractive forcecontrol;

FIG. 10 is a side view of a working implement showing definition of boomangle;

FIG. 11 shows the speed of change of instruction values for the motordisplacement when changing the motor displacement;

FIG. 12 is a flowchart showing the processes for determining whether ornot a work phase is excavation;

FIG. 13 is a flow chart showing the processes for determining whether ornot a boom pressure lower flag is ON; and

FIG. 14 is a flowchart showing the processes for determining whether ornot the working implement is in a raising hydraulic stall condition.

DESCRIPTION OF EMBODIMENTS

A wheel loader 50 according to a first embodiment of the presentinvention will now be described with reference to the drawings. FIG. 1is a side view of the wheel loader 50. The wheel loader 50 includes abody 51, a working implement 52, a plurality of tires 55, and a cab 56.The working implement 52 is installed at the front part of the body 51.The working implement 52 includes a boom 53, a bucket 54, a liftcylinder 19 and a bucket cylinder 26. The boom 53 is a member used forlifting up the bucket 54. The boom 53 is driven by the lift cylinder 19.The bucket 54 is attached at the end of the boom 53. The bucket 54 canbe made to dump or tilt by the bucket cylinder 26. The bucket cylinder26 raises the bucket 54 by moving the boom 53. The cab 56 is placed inposition over the body 51.

FIG. 2 is a block diagram of the configuration of a hydraulic drivemechanism 30 mounted on the wheel loader 50. A hydraulic drive mechanism30 includes chiefly, an engine 1, a first hydraulic pump 4, a secondhydraulic pump 2, a charge pump 3, a traveling hydraulic motor 10, anengine controller 12 a, a vehicle controller 12 and a drive hydrauliccircuit 20. In the hydraulic drive mechanism 30, the first hydraulicpump 4 discharges hydraulic fluid as it is driven by the engine 1. Thetraveling hydraulic motor 10 is driven by hydraulic fluid dischargedfrom the first hydraulic pump 4. The traveling hydraulic motor 10 movesthe wheel loader 50 by driving the above-mentioned tires 55 to rotate.That is to say, the hydraulic drive mechanism 30 employs what is calleda 1 pump 1 motor HST system.

The engine 1 is a diesel engine. Output torque generated at the engine 1is conveyed to the second hydraulic pump 2, the charge pump 3 and thefirst hydraulic pump 4. An engine rotation speed sensor 1 a that detectsactual rotation speed of the engine 1 is installed in the hydraulicdrive mechanism 30. The engine rotation speed sensor 1 a is an exampleof the engine rotation speed detection part of the present invention.Further, a fuel injection device 1 b is connected to the engine 1. Theengine controller 12 a described subsequently, controls the rotationspeed and output torque (hereinafter “engine torque”) of the engine 1 bycontrolling the fuel injection device 1 b.

The first hydraulic pump 4 discharges hydraulic fluid as it is driven bythe engine 1. The first hydraulic pump 4 is a variable displacement typehydraulic pump. Hydraulic fuel discharged from the first hydraulic pump4 passes the drive hydraulic circuit 20 and is delivered to thetraveling hydraulic motor 10. Basically, the drive hydraulic circuit 20includes a first drive circuit 20 a and a second drive circuit 20 b. Ashydraulic fluid is supplied to the traveling hydraulic motor 10 from thefirst hydraulic pump 4 via the first drive circuit 20 a, the travelinghydraulic motor 10 is driven in one direction (e.g. the forwarddirection). As hydraulic fluid is supplied to the traveling hydraulicmotor 10 from the first hydraulic pump 4 via the second drive circuit 20b, the traveling hydraulic motor 10 is driven in the other direction(e.g. the reverse direction).

A drive circuit pressure detection part 17 is installed in the drivehydraulic circuit 20. The drive circuit pressure detection part 17detects the pressure of hydraulic fluid (hereinafter “drive circuitpressure”) supplied to the traveling hydraulic motor 10 via the firstdrive circuit 20 a or the second drive circuit 20 b. Basically, thedrive circuit pressure detection part 17 includes a first drive circuitpressure sensor 17 a and a second drive circuit pressure sensor 17 b.The first drive circuit pressure sensor 17 a detects the hydraulicpressure of the first drive circuit 20 a. The second drive circuitpressure sensor 17 b detects the hydraulic pressure of the second drivecircuit 20 b. The first drive circuit pressure sensor 17 a and thesecond drive circuit pressure sensor 17 b send detection signals to thevehicle controller 12. Further, an FR switching part 5 and a pumpdisplacement control cylinder 6 are connected to the first hydraulicpump 4 for controlling the direction of discharge from the firsthydraulic pump 4.

The FR switching part 5 is an electromagnetic control valve thatswitches the direction of hydraulic fluid supplied to the pumpdisplacement control cylinder 6 based on a control signal from thevehicle controller 12. The FR switching part 5, by switching thedirection in which hydraulic oil is supplied to the pump displacementcontrol cylinder 6, switches the direction of discharge from the firsthydraulic pump 4. Basically, the FR switching part 5 switches thedirection of discharge from the first hydraulic pump 4 between dischargeto the first drive circuit 20 a and discharge to the second drivecircuit 20 b. In this way the drive direction of the traveling hydraulicmotor 10 is changed. The pump displacement control cylinder 6 is drivenby hydraulic oil supplied via a pump pilot circuit 32 and changes thetilting angle of the first hydraulic pump 4.

A pump displacement control part 7 is arranged in the pump pilot circuit32. The pump displacement control part 7 connects the pump displacementcontrol cylinder 6 to any of the pump pilot circuit 32 and a hydraulicfluid tank. The pump displacement control part 7 is an electromagneticcontrol valve controlled based on control signals from the vehiclecontroller 12. The pump displacement control part 7 adjusts the tiltingangle of the first hydraulic pump 4 by controlling the pressure ofhydraulic fluid inside the pump displacement control cylinder 6.

The pump pilot circuit 32 is connected to a charge circuit 33 and ahydraulic fluid tank via a cutoff valve 47. The pilot port of the cutoffvalve 47 is connected to the first drive circuit 20 a and the seconddrive circuit 20 b via a shuttle valve 46. The shuttle valve 46introduces whichever hydraulic pressure is greatest between the firstdrive circuit 20 a and the second drive circuit 20 b to the pilot portof the cutoff valve 47. That is to say, drive circuit pressure isapplied to the pilot port of the cutoff valve 47. The cutoff valve 47links the charge circuit 33 and the pump pilot circuit 32 when drivecircuit pressure is below a predetermined cutoff pressure. In this wayhydraulic fluid is supplied from the charge circuit 33 to the pump pilotcircuit 32. When drive circuit pressure is above the predeterminedcutoff pressure the cutoff valve 47 links the pump pilot circuit 32 to ahydraulic fluid tank and releases hydraulic fluid of the pump pilotcircuit 32 to the hydraulic fluid tank. In this way, the displacement ofthe first hydraulic pump 4 is reduced by decrease in pressure of thepump pilot circuit 32, preventing a rise in drive circuit pressure.

The charge pump 3, driven by the engine 1, is a pump for supplyinghydraulic fluid to the drive hydraulic circuit 20. The charge pump 3 isconnected to the charge circuit 33. The charge pump 3 supplies hydraulicfluid to the pump pilot circuit 32 via the charge circuit 33. The chargecircuit 33 is connected to the first drive circuit 20 a via a firstcheck valve 41. The first check valve 41 allows flow of hydraulic fluidfrom the charge circuit 33 to the first drive circuit 20 a but restrictsflow of hydraulic fluid from the first drive circuit 20 a to the chargecircuit 33. Further, the charge circuit 33 is connected to the seconddrive circuit 20 b via a second check valve 42. The second check valve42 allows flow of hydraulic fluid from the charge circuit 33 to thesecond drive circuit 20 b but restricts flow of hydraulic fluid from thesecond drive circuit 20 b to the charge circuit 33. Again, the chargecircuit 33 is connected to the first drive circuit 20 a via a firstrelief valve 43. The first relief valve 43 opens when the hydraulicpressure of the first drive circuit 20 a is greater than a predeterminedpressure. The charge circuit 33 is connected to the second drive circuit20 b via a second relief valve 44. The second relief valve 44 opens whenthe hydraulic pressure of the second drive circuit 20 b is greater thana predetermined pressure. Further, the charge circuit 33 is connected toa hydraulic fluid tank via a low pressure relief valve 45. The lowpressure relief valve 45 opens when the hydraulic pressure of the chargecircuit 33 is greater than a predetermined relief pressure. In this way,drive circuit pressure is adjusted so as not to exceed a predeterminedrelief pressure. The predetermined relief pressure of the low pressurerelief valve 45 is low in comparison to the relief pressure of the firstrelief valve 43 and the relief pressure of the second relief valve 44.Accordingly, when drive circuit pressure is lower than hydraulicpressure of the charge circuit 33, hydraulic fluid is supplied from thecharge circuit 33 to the drive hydraulic circuit 20 via the first checkvalve 41 or the second check valve 42.

The second hydraulic pump 2 is driven by the engine 1. The secondhydraulic pump 2 discharges hydraulic fluid to drive the lift cylinder19. Hydraulic fluid discharged from the second hydraulic pump 2 issupplied to the lift cylinder 19 via a working implement hydrauliccircuit 31. Thereby the working implement 52 is driven. The dischargepressure of the second hydraulic pump 2 is detected by a dischargepressure sensor 39. The discharge pressure sensor 39 sends detectionsignals to the vehicle controller 12. A working implement control valve18 is installed in the working implement hydraulic circuit 31. Theworking implement control valve 18 is driven in compliance with theoperation amount of a working implement operating member 23. The workingimplement operating member 23 is a member for operating the workingimplement 52. The working implement control valve 18 controls the flowrate of hydraulic fluid supplied to the lift cylinder 19 in compliancewith pilot pressure applied to a pilot port. Pilot pressure applied tothe pilot port of the working implement control valve 18 is controlledby a pilot valve 23 a of the working implement operating member 23. Thepilot valve 23 a applies pilot pressure to the pilot port of the workingimplement control valve 18 in compliance with the operation amount ofthe working implement operating member 23. In this way, the liftcylinder 19 is controlled in compliance with the operation amount of theworking implement operating member 23. Pilot pressure applied to thepilot port of the working implement control valve 18 is detected by aPPC pressure sensor 21. The pressure of hydraulic fluid supplied to thelift cylinder 19 is detected by a boom pressure sensor 22. The boompressure sensor 22 is an example of the boom pressure detection part ofthe present invention. The PPC pressure sensor 21 and the boom pressuresensor 22 send detection signals to the vehicle controller 12. A boomangle detection part 38 is attached to the lift cylinder 19. The boomangle detection part 38 detects the angle of the boom as describedsubsequently. The boom angle detection part 38 is a sensor for detectingthe angle of rotation of the boom 53. Alternatively, it is also suitablefor the boom angle detection part 38 to detect the length of stroke ofthe lift cylinder 19 such that the rotation angle of the boom 53 can becalculated from that length of stroke. The boom angle detection part 38sends detection signals to the vehicle controller 12. The bucketcylinder 26 is also controlled by a control valve in the same manner asthe lift cylinder 19, however this valve is omitted from FIG. 2.

The traveling hydraulic motor 10 is a variable displacement typehydraulic motor. The traveling hydraulic motor 10 is driven by hydraulicfluid discharged from the first hydraulic pump 4, to generate driveforce that facilitates travel. A motor cylinder 11 a and motordisplacement control part 11 b are installed to the traveling hydraulicmotor 10. The motor cylinder 11 a changes the tilting angle of thetraveling hydraulic motor 10. The motor displacement control part 11 bis an electromagnetic valve controlled based on control signals from thevehicle controller 12. The motor displacement control part 11 b controlsthe motor cylinder 11 a based on control signals from the vehiclecontroller 12. The motor cylinder 11 a and the motor displacementcontrol part 11 b are connected to a motor pilot circuit 34. The motorpilot circuit 34 is connected to the first drive circuit 20 a via acheck valve 48. The check valve 48′ allows flow of hydraulic fluid fromthe first drive circuit 20 a to the motor pilot circuit 34 but restrictsflow of hydraulic fluid from the motor pilot circuit 34 to the firstdrive circuit 20 a. The motor pilot circuit 34 is connected to thesecond drive circuit 20 b via a check valve 49. The check valve 49allows flow of hydraulic fluid from the second drive circuit 20 b to themotor pilot circuit 34 but restricts flow of hydraulic fluid from themotor pilot circuit 34 to the second drive circuit 20 b. Throughoperation of the check valve 48 and the check valve 49, whicheverhydraulic pressure is greatest between the first drive circuit 20 a andthe second drive circuit 20 b, in other words drive circuit pressurehydraulic fluid, is supplied to the motor pilot circuit 34. The motordisplacement control part 11 b switches the supply flow volume anddirection of supply of hydraulic fluid to the motor cylinder 11 a fromthe motor pilot circuit 34, based on control signals from the vehiclecontroller 12. In this way, the vehicle controller 12 can freely changethe displacement of the traveling hydraulic motor 10. Further, itenables the upper limit displacement and lower limit of displacement ofthe traveling hydraulic motor 10 to be set as required.

A vehicle speed sensor 16 is installed on the hydraulic drive mechanism30. The vehicle speed sensor 16 detects vehicle speed. The vehicle speedsensor 16 sends detection signals to the vehicle controller 12. Thevehicle speed sensor 16 detects vehicle speed by for example, detectingthe rotation speed of the tire drive shaft.

The wheel loader 50 includes an accelerator operation member 13 a, aforward/reverse control member 14, a tractive force operating member 15,an inching operation part 27 and a settings operation device 24.

The accelerator operation member 13 a is a member that enables anoperator to set a target rotation speed for the engine 1. Theaccelerator operation member 13 a is for example, an accelerator pedaloperated by the operator. The accelerator operation member 13 a isconnected to an accelerator operation sensor 13. The acceleratoroperation sensor 13 is comprised for example as a potentiometer. Theaccelerator operation sensor sends detection signals showing operationamount of the accelerator operation member 13 a (hereinafter“accelerator operation amount”) to the engine controller 12 a. Theoperator, by adjusting the accelerator operation amount is able tocontrol the rotation speed of the engine 1.

The forward/reverse control member 14 is operated to switch thedirection of travel of the vehicle. The forward/reverse control member14 is operated by the operator switching between a forward travelposition, a reverse travel position and a neutral position. Theforward/reverse control member 14 sends detection signals showing theposition of the forward/reverse control member 14 to the vehiclecontroller 12. The operator, by operating the forward/reverse controlmember 14, is able to switch between a forward direction and reversedirection of travel of the wheel loader 50. The tractive force operatingmember 15 is for example, a switch. The tractive force operating member15 is operated by the operator for switching tractive force control ONor OFF. The tractive force control is a control for reducing maximumtractive force of the wheel loader 50. In the following explanation, thetractive force control in the OFF condition means the condition in whichthe tractive force control is not being implemented. The tractive forcecontrol in the ON condition means the condition in which the tractiveforce control is being implemented. A more detailed explanation of thetractive force control is provided subsequently. The tractive forceoperating member 15 sends detection signals showing the selectedposition of the tractive force operating member 15 to the vehiclecontroller 12.

The inching operation part 27 includes an inching operation member 27 aand an inching operation sensor 27 b. The inching operation member 27 ais operated by an operator. The inching operation member 27 a is forexample, a pedal. The inching operation member 27 a provides bothinching operation functions and braking operation functions as describedsubsequently. The inching operation sensor 27 b detects operation amountof the inching operation member 27 a (hereinafter “inching operationamount”) and sends detection signals to the vehicle controller 12. Whenthe inching operation member 27 a is operated, the vehicle controller 12controls the pump displacement control part 7 based on the detectionsignal from the inching operation sensor 27 b. The vehicle controller 12lowers hydraulic pressure of the pump pilot circuit 32 in compliancewith the operation amount of the inching operation member 27 a. In thisway, drive circuit pressure is lowered and the rotation speed of thetraveling hydraulic motor 10 decreases. The inching operation part 27 isused when for example the operator wishes to raise the rotation speed ofthe engine 1 while suppressing increase in travel speed. That is to say,if the rotation speed of the engine 1 is raised through operation of theaccelerator operation member 13 a, the hydraulic pressure of the pumppilot circuit 32 rises also. Here, by operating the inching operationmember 27 a the operator can control rise in the hydraulic pressure ofthe pump pilot circuit 32. In this way increase in the displacement ofthe first hydraulic pump 4 is prevented enabling increase in therotation speed of the traveling hydraulic motor 10 to be prevented. Inother words, the inching operation member 27 a is operated in order tolower vehicle speed or tractive force without lowering engine rotationspeed.

Further, a brake valve 28 is linked to the inching operation member 27a. The brake valve 28 controls supply of hydraulic fluid to a hydraulicbrake device 29. The inching operation member 27 a thus also operates asa member for operating the hydraulic brake device 29. Until the inchingoperation amount of the inching operation member 27 a reaches apredetermined amount, only the above described inching operation basedon detection signals from the inching operation sensor 27 b will beperformed. When the inching operation amount of the inching operationmember 27 a reaches the predetermined amount, operation of the brakevalve 28 commences, thereby generating braking force for the hydraulicbrake device 29. When the inching operation amount of the inchingoperation member 27 a is greater than or equal to a predeterminedamount, braking force of the hydraulic brake device 29 is controlled incompliance with the inching operation amount of the inching operationmember 27 a.

The settings operation device 24 is a device for performing variouskinds of settings for the wheel loader 50. The settings operation device24 is for example a display device with touch panel functionality. Asdescribed subsequently, in the tractive force control a control level oftractive force can be set at a first level. The maximum tractive forceof the first level is less than the maximum tractive force with thetractive force control in the OFF condition. An operator, by operatingthe settings operation device 24, can select from a plurality of levelsfor the magnitude of maximum tractive force of the first level using thetractive force control, and set that selected level. The settingsoperation device 24 is an example of a tractive force level changingpart for changing the magnitude of maximum tractive force of the firstlevel.

The engine controller 12 a is an electronic control part providing anarithmetic unit such as a CPU, or various kinds of memory. The enginecontroller 12 a controls the engine 1 so as to obtain the set targetrotation speed. FIG. 3 shows the output torque line of the engine 1. Theoutput torque line of the engine 1 shows the relationship betweenrotation speed of the engine 1 and the upper limit of engine torque theengine 1 can output for each rotation speed. In FIG. 3 the solid lineL100 shows the engine output torque line when the accelerator operationamount is 100%. This engine output torque line is for example,equivalent to the rated value of the engine 1 or maximum power output.Accelerator operation amount at 100% means the condition in which theaccelerator operation member 13 a is shifted to the maximum extent.Again, the short dashed line L75 shows the engine output torque linewhen the accelerator operation amount is 75%. The engine controller 12 acontrols output of the engine 1 such that engine torque is below theengine output torque line. This engine 1 output control is for exampleperformed by controlling the upper value of the quantity of fuelinjected to the engine 1.

The vehicle controller 12 is an electronic control part providing anarithmetic unit such as a CPU, or various kinds of memory. The vehiclecontroller 12 controls the displacement of the first hydraulic pump 4and the displacement of the traveling hydraulic motor 10 by electroniccontrol of each control valve based on detection signals from eachdetection part.

Basically, the vehicle controller 12 outputs an instruction signal tothe pump displacement control part 7 based on engine rotation speeddetected by the engine rotation speed sensor 1 a. In this way, therelationship between pump displacement and drive circuit pressure isregulated. FIG. 4 shows an example of the characteristics of pumpdisplacement and drive circuit pressure. The characteristics of pumpdisplacement and drive circuit pressure are the relationship betweenpump displacement and drive circuit pressure. L11-L16 in FIG. 4 arelines that show the changing characteristics of pump displacement inrelation to drive circuit pressure in compliance with engine rotationspeed. Basically, as the vehicle controller 12 controls the flow volumeof the pump displacement control part 7 based on the engine rotationspeed, the relationship of pump displacement to drive circuit pressurechanges as indicated by L11-L16. In this way, pump displacement iscontrolled to comply with engine rotation speed and drive circuitpressure.

The vehicle controller 12 processes detection signals from the enginerotation speed sensor 1 a and the drive circuit pressure detection part17 and outputs motor displacement instruction signals to the motordisplacement control part 11 b. Here, the vehicle controller 12references the characteristics of motor displacement-drive circuitpressure stored in the vehicle controller 12 and sets motor displacementfrom the value for engine rotation speed and the value for drive circuitpressure. The vehicle controller 12 outputs instructions to changetilting angle in relation to motor displacement as set to the motordisplacement control part 11 b. FIG. 5 shows an example of thecharacteristics of motor displacement to drive circuit pressure. In FIG.5 the solid line L21 is the line that defines motor displacement inrelation to drive circuit pressure for conditions representing thevalues of engine rotation speed. Here, motor displacement corresponds tothe tilting angle of the traveling hydraulic motor 10. Until drivecircuit pressure falls below a certain value, tilting angle is minimum(Min). Thereafter tilting angle increases gradually as drive circuitpressure increases (the inclined portion of solid line L22). After thetilting angle reaches the maximum (Max), maximum tilting angle (Max) ismaintained even though drive circuit pressure rises. The inclinedportion L22 defines drive circuit pressure target pressure. In otherwords, the vehicle controller 12 increases the displacement of thetraveling hydraulic motor if drive circuit pressure becomes greater thantarget pressure. Again, if drive circuit pressure falls below the targetpressure, the vehicle controller 12 lowers the displacement of thetraveling hydraulic motor. The target pressure is defined in conformancewith engine rotation speed. In other words, the inclined portion L22 inFIG. 5 is determined as rising or falling in compliance with theincrease or decrease in engine rotation speed. Basically, the inclinedportion L22 shows that if engine rotation speed is low, tilting angleincreases from the condition in which drive circuit pressure is lower,controlled such that maximum tilting angle is reached with drive circuitpressure in a lower condition (in FIG. 5, the inclined dashed line L23in the lower part). In the opposite case, if engine rotation speed ishigh, the minimum tilting angle (Min) is maintained until drive circuitpressure becomes higher, controlled such that maximum tilting angle(Max) is reached with drive circuit pressure in a higher condition (inFIG. 5, dashed line L24 in the upper part). In this way, as shown inFIG. 6, in the wheel loader 50 tractive force and speed changeseamlessly without stages, enabling automatic speed change from zero tomaximum speed with no speed change operation. The inclined portion L22in FIG. 5 is shown with the inclination emphasized for ease ofunderstanding, but actually should be almost horizontal. Accordingly, ifdrive circuit pressure reaches target pressure, motor displacementswitches between a minimum value (minimum limiting value) and maximumvalue (maximum limiting value). However, when drive circuit pressure hasreached target pressure the instructed value is not changed immediatelyand a time delay occurs. This time delay is the reason the inclinedportion L22 exists.

The vehicle controller 12 performs the tractive force control byoperation of the tractive force operating member 15. The vehiclecontroller 12 changes tractive force of the vehicle by changing theupper limit displacement of the traveling hydraulic motor 10. As shownin FIG. 5 for example, the vehicle controller 12 outputs an instructionsignal to the motor displacement control part 11 b in order to changeupper limit displacement from Max to any of Ma, Mb or Mc. When the upperlimit displacement is changed to Ma, the characteristics of vehiclespeed to tractive force change as shown by the line La in FIG. 6. Inthis way maximum tractive force is lowered in comparison to the line L1that shows vehicle speed to tractive force in the condition in which thetractive force control is not being implemented. When upper limitdisplacement is changed to Mb, the characteristics of vehicle speed totractive force change as indicated by line Lb, as maximum tractive forcereduces further. Again, if the upper limit displacement is changed toMc, the characteristics of vehicle speed to tractive force change asindicated by the line Lc as maximum tractive force decreases stillfurther.

Under the tractive force control maximum tractive force of the vehicleis reduced to maximum tractive force of the first level set in advance.The settings operation device 24 described above, can select from aplurality of levels for the magnitude of maximum tractive force of thefirst level for the tractive force control and set that level.Basically, the settings operation device 24 can select from three gradesof level, Level A, Level B or Level C to set the first level. Level A,is the level of tractive force corresponding to upper limit displacementMa above. Level B is the level of tractive force corresponding to upperlimit displacement Mb above. Level C is the level of tractive forcecorresponding to upper limit displacement Mc above.

FIG. 7 shows an example of tractive force ratio information defining therelationship of tractive force ratio to accelerator operation amount.The tractive force ratio shows the ratio of maximum tractive force underthe tractive force control when maximum tractive force with the tractiveforce control in the OFF condition is 100%. In FIG. 7 Lv1 is tractiveforce ratio information of the first level (hereinafter “first tractiveforce ratio information”). For the first tractive force ratioinformation Lv1, when the accelerator operation amount is less than orequal to a predetermined threshold A, the tractive force ratio isconstant at R1. When the accelerator operation amount rises above thepredetermined threshold A, the tractive force ratio increases incompliance with accelerator operation amount. The vehicle controller 12controls the upper limit displacement of the traveling hydraulic motor10 so as to achieve maximum tractive force as shown by the firsttractive ratio information Lv1 when the control level of tractive forceis set to the first level for the tractive force control.

The vehicle controller 12 changes the control level of tractive forcefrom the first level to the second level when predetermineddetermination conditions are satisfied during the tractive forcecontrol. In FIG. 7 Lv2 is tractive force ratio information of the secondlevel (hereinafter “second tractive force ratio information”). Thetractive force ratio of the second level Lv2 is smaller than thetractive force ratio of the first level Lv1. Tractive force ratio of thesecond level Lv2 is less than tractive force ratio of the first levelLv1 by a predetermined amount of change dR. Amount of change dR shouldpreferably be not less than 5% and not greater than 15%. Amount ofchange dR should be for example 10%. The vehicle controller 12 controlsthe upper limit displacement of the traveling hydraulic motor 10 so asto achieve maximum tractive force as shown by the second tractive ratioinformation Lv2, when the determination conditions are satisfied duringthe tractive force control. A detailed description of the determinationprocess for automatically lowering tractive force for the tractive forcecontrol will now be provided.

As shown in FIG. 8, the vehicle controller 12 includes a the tractiveforce control part 61, a work phase determination part 62, a workingimplement raising hydraulic stall determination part 63, a boom angledetermination part 64, a drive circuit pressure determination part 65,an engine rotation speed determination part 66 and a change flagdetermination part 67. FIG. 9 is a flowchart showing the determinationprocess for changing the control level of tractive force from the firstlevel to the second level during the tractive force control. The vehiclecontroller 12 implements the processes shown in FIG. 9 when the tractiveforce control is set to the ON condition by operation of the tractiveforce operating member 15.

At step S101 the tractive force control part 61 sets the control levelof tractive force at the first level. At step S102 the tractive forcecontrol part 61 sets the change flag to OFF. The change flag is set toON when the tractive force control level is lowered from the first levelto the second level. The change flag is set to OFF when the tractiveforce control level is not lowered from the first level to the secondlevel. That is to say, when the change flag is OFF, the tractive forcecontrol part 61 maintains the control level of tractive force at thefirst level.

Next, at step S103, the work phase determination part 62 determineswhether or not the excavation flag is ON. The excavation flag being ONmeans that the work phase is excavation. As described subsequently, thework phase determination part 62 determines whether or not the workphase is excavation based on the condition of travel of the vehicle andthe operating state of the working implement 52. The work phasedetermination part 62 sets the excavation flag to ON when thedetermination is that the work phase is excavation. The work phasedetermination part 62 sets the excavation flag to OFF when thedetermination is that the work phase is work other than excavation. Amore specific explanation of the process for determination of work phaseis provided subsequently.

At step S104, the working implement raising hydraulic stalldetermination part 63 determines whether or not the working implementraising hydraulic stall flag is ON. The working implement raisinghydraulic stall flag being ON means that the working implement 52 is inthe raising hydraulic stall condition. The raising hydraulic stallcondition is a condition in which the boom 53 does not move even whenthe working implement operating member 23 is operated to raise the boom.As described subsequently, the working implement raising hydraulic stalldetermination part 63 determines whether or not the working implement 52is in the raising hydraulic stall condition based on the operationamount of the working implement operating member 23 and boom pressure asdescribed subsequently. When the working implement raising hydraulicstall determination part 63 determines that the working implement 52 isin the raising hydraulic stall condition, it sets the working implementraising hydraulic stall flag to ON. The working implement raisinghydraulic stall determination part 63 sets the raising hydraulic stallflag to OFF if it determines that the working implement is not in theraising hydraulic stall condition. A description of the fundamentalprocesses for determining whether or not the working implement is in theraising hydraulic stall condition is provided subsequently.

At step S105, the boom angle determination part 64 determines whether ornot the boom angle is below a predetermined angle threshold B1. The boomangle determination part 64 makes this determination based on detectionsignals from the boom angle detection part 38. As shown in FIG. 10providing a side view, the boom angle is the angle θ between thehorizontal direction and the line joining the boom pin 57 and the bucketpin 58, the horizontal direction being 0°. An angle below the horizontaldirection is a minus value and an angle above the horizontal directionis a plus value. The boom angle is defined so as to increase toward theupward direction. The angle threshold B1 corresponds to the boom anglewith the bucket 54 on the ground surface. For example, the anglethreshold B1 is −10°. It is preferable for the angle threshold B1 to benot greater than −20°. The boom angle being below the predeterminedangle threshold B1 means that the bucket 54 is in a condition in whichit cannot be raised when pushed into the subject material such as earthand sand and the like.

At step S106 the drive circuit pressure determination part 65 determineswhether or not drive circuit pressure is above a predetermined hydraulicpressure threshold C1. The drive circuit pressure determination part 65makes this determination based on detection signals from the drivecircuit pressure detection part 17. The hydraulic pressure threshold C1is a value of a degree that enables it to be deemed that there issufficient tractive force for the purpose of performing a scooping inoperation.

At step S107, the engine rotation speed determination part 66 determineswhether or not engine rotation speed is above a predetermined rotationspeed threshold D1. The engine rotation speed determination part 66makes the determination based on detection signals from the enginerotation speed sensor 1 a. The rotation speed threshold D1 is a valuesufficient to prevent a sudden fall in engine rotation speed when thecontrol level of tractive force is lowered from the first level to thesecond level. As shown in FIG. 3, for the output torque line, the enginerotation speed threshold D1 is the engine rotation speed when the upperlimit of engine torque is the maximum value Tmax, that is to say,maximum torque rotation speed.

If at least one of the conditions among the conditions from step S103 tostep S107 is not satisfied, step S108 is implemented. At step S108 thetractive force control part 61 sets the tractive force control level atthe first level. That is to say, if at least one of the conditions fromstep S103 through step S107 is not satisfied in the condition in whichthe control level of tractive force is at the first level, the tractiveforce control level is maintained at the first level. If at least one ofthe conditions from among the conditions from step S103 through stepS107 is not satisfied in the condition in which the control level oftractive force is at the second level, the control level of tractiveforce returns from the second level to the first level. Accordingly, thetractive force control part 61 does not lower maximum tractive forcewhen the work phase is not excavation. The tractive force control part61 does not lower maximum tractive force when the working implement 52is in the raising hydraulic stall condition. The tractive force controlpart 61 does not lower maximum tractive force when drive circuitpressure is not greater than or equal to the predetermined hydraulicpressure threshold C1. The tractive force control part 61 does not lowermaximum tractive force when the boom angle is not less than or equal tothe predetermined angle threshold B1. The tractive force control part 61does not lower maximum tractive force when the engine rotation speed isnot greater than or equal to the engine rotation threshold D1.

When all of the conditions from step S103 to step S107 are satisfied,step S109 is performed. At step S109 the change flag determination part67 determines whether or not the change flag is OFF. That is to say, thechange flag determination part 67 determines whether or not the tractiveforce control level is at the first level. If the change flag is OFF, inother words, when the tractive force control level is the first level,step S110 is performed.

At step S110 the tractive force control part 61 sets the change flag toON. Further, at step S111 the tractive force control part 61 changes thecontrol level of tractive force from the first level to the secondlevel. In this way, the tractive force control part 61 controls maximumtractive force based on the second tractive force ratio information Lv2shown in FIG. 7. In this way, maximum tractive force is lowered.

At step S109, if the change flag is not OFF, the tractive force controllevel is maintained at the second level and the determinations of stepS103 to step S109 are repeated. If any of the conditions from step S103to step S107 have ceased being satisfied, the control level of tractiveforce returns from the second level to the first level.

The tractive force control part 61, when returning the tractive forcecontrol level from the second level to the first level, makes tractiveforce change more slowly than when lowering from the first level to thesecond level. That is to say, when the tractive force control part 61increases maximum tractive force during the tractive force control, itmakes maximum tractive force change more slowly than when loweringmaximum tractive force. FIG. 11 (a) shows the speed of change ofinstruction values of motor displacement when motor displacement is madeto increase. FIG. 11 (b) shows the speed of change of instruction valuesof motor displacement when motor displacement is made to reduce. Asshown in FIG. 11 time T1 is greater than time T2. Accordingly, thetractive force control part 61 when increasing maximum tractive force,changes instruction values for motor displacement more slowly than whenreducing maximum tractive force.

FIG. 12 is a flowchart showing the processes for determining whether ornot the work phase is excavation. As shown in FIG. 12, at step S201 thework phase determination part 62 sets the excavation flag to OFF. Atstep S202 the work phase determination part 62 determines whether or notthe boom pressure lower flag is ON. The boom pressure lower flag beingON means that the bucket is in the unladen condition. The processes fordetermination of the status of the boom pressure lower flag aredescribed subsequently.

At step S203 a determination is made as to whether or not the boom angleis less than the predetermined angle threshold B2. The angle thresholdB2 corresponds to the boom angle when the bucket is positioned on theground surface. The angle threshold B2 may be the same as the anglethreshold B1 described above.

At step S204 the work phase determination part 62 determines whether ornot boom pressure is greater than a first boom pressure determinationvalue. Boom pressure is the pressure supplied to the lift cylinder 19when the lift cylinder 19 is made to extend. Boom pressure is detectedby the boom pressure sensor 22 described above. The first boom pressuredetermination value is a value for boom pressure obtainable duringexcavation. A predefined value obtained from experiment or simulation isset as the first boom pressure determination value. The first boompressure determination value is a value in compliance with the boomangle. The vehicle controller 12 has stored, boom pressure determinationvalue information that shows the relationship between the first boompressure value and boom angle (hereinafter “first boom pressure valueinformation”). The first boom pressure value information is for example,a table or map showing the relationship between the first boom pressuredetermination value and boom angle. The work phase determination part 62determines the first boom pressure determination value in conformancewith boom angle by referencing the first boom pressure valueinformation.

If all the conditions from step S202 to step S204 are satisfied, stepS205 is performed. At step S205 the work phase determination part 62sets the excavation flag to ON. That is to say, the work phasedetermination part 62 determines that the work phase is excavation whenall of the conditions from step S202 to step S204 are satisfied. This isbecause when all of the conditions from step S202 to step S204 aresatisfied, it can be understood that the wheel loader 50 has entered theexcavation preparation stage. If at least one of the conditions fromsteps S202, S203 or S204 are not satisfied, the determinations from stepS202 to step S204 are repeated.

Further, at step S206, the work phase determination part 62 sets theboom pressure lower flag to OFF. Then, at step S207, the work phasedetermination part 62 determines whether or not the FNR recognitionvalue is F. The FNR recognition value is information showing which ofconditions, a forward travel condition, a reverse travel condition and aneutral condition, is a condition of the vehicle. The FNR recognitionvalue being F means that the vehicle is in the forward travel condition.The FNR recognition value being R means that the vehicle is in thereverse travel condition. The FNR recognition value being N means thatthe vehicle is in the neutral condition. The work phase determinationpart 62 determines whether or not the FNR recognition value is F basedon detection signals from the forward/reverse control member 14. Whenthe FNR recognition value is not F, step S209 is performed. At step S209the work phase determination part 62 sets the excavation flag to OFF.That is to say when the vehicle is in the reverse travel condition or inthe neutral condition, the excavation flag is set to OFF. At step S207when the FNR recognition value is F, step S208 is proceeded to.

At step S208, the work phase determination part 62 determines whether ornot the boom pressure lower flag is ON. If the boom pressure lower flagis ON, step S209 is implemented. If the boom pressure lower flag is notON, step S207 is returned to. Accordingly, once the determination ismade that the work phase is excavation, thereafter the excavation flagis maintained at ON until either the forward/reverse control member 14switches from the forward position to the reverse position or theforward/reverse control member 14 switches from the forward position tothe neutral position, even if the conditions from step S202 to step S204cease being satisfied. Note that even if the forward/reverse controlmember 14 is maintained in the forward position, when the boom pressurelower flag is set to ON, the excavation flag changes to OFF.

FIG. 13 is a flow chart showing the processes for determining whether ornot the boom pressure lower flag is ON. As shown in FIG. 13, at stepS301 the work phase determination part 62 sets the boom pressure lowerflag to OFF.

At step S302, the work phase determination part 62 commencesmeasurements of a first timer. Here the first timer measures thecontinuous time during which the conditions for setting the boompressure lower flag to ON are satisfied.

At step S303 the work phase determination part 62 determines whether ornot boom pressure is below a second boom pressure determination value.The second boom pressure determination value is the obtainable boompressure when the bucket is in the unladen condition. The vehiclecontroller 12 has stored, boom pressure determination value informationthat shows the relationship between the second boom pressuredetermination value and boom angle (hereinafter “second boom pressurevalue information”). The second boom pressure determination valueinformation is for example, a table or map showing the relationshipbetween the second boom pressure determination value and boom angle. Thework phase determination part 62 determines the second boom pressuredetermination value in conformance with boom angle by referencing thesecond boom pressure value information. With the second boom pressurevalue information, when the boom angle is greater than 0°, the secondboom pressure determination value is consistent with the value when theboom angle is at 0°. The ratio of increase of boom pressure when theboom angle is greater than or equal to 0° is less than the ratio ofincrease of boom pressure when the boom angle is less than 0°, thus thesecond boom pressure determination value when the boom angle is greaterthan 0° can be proximate to the second boom pressure determination valuewhen the boom angle is at 0°.

At step S304 the work phase determination part 62 determines whether ornot time measured by the first timer is greater than or equal to apredetermined time threshold E1. That is to say, a time durationdetermination part 67 determines whether or not the time duration of thecondition in which the conditions of step S303 are satisfied is greaterthan or equal to a predetermined time threshold E1. The time thresholdE1 is set to be a time sufficient to enable it to be deemed that theconditions of step S303 are temporarily not being satisfied. If the timemeasured by the first timer is not greater than or equal to thepredetermined time threshold E1 the determinations of step S303 arerepeated. At step S304, if the time measured by the first timer isgreater than or equal to the predetermined time threshold E1, step S305is then proceeded to.

At step S305 the work phase determination part 62 sets the boom pressurelower flag to ON. Then at step S306, the work phase determination part62 finishes measurements of the first timer. Note that at step S303, ifboom pressure is not less than the second boom pressure determinationvalue, step S307 is proceeded to. At step S307 the work phasedetermination part 62 resets the first timer.

At step S308, the work phase determination part 62 commencesmeasurements of a second timer. Then, at step S309 the work phasedetermination part 62 determines whether or not the excavation flag isON. If the excavation flag is ON, step S310 is performed.

At step S310 the work phase determination part 62 finishes themeasurements of the second timer. Then the processing returns to stepS301 where the work phase determination part 62 sets the boom pressurelower flag to OFF.

At step S309, if the excavation flag is not ON, step S311 is performed.At step S311 the work phase determination part 62 determines whether ornot the boom pressure is less than the second boom pressuredetermination value. If the boom pressure is less than the second boompressure determination value, step S312 is performed.

At step S312 the work phase determination part 62 determines whether ornot time measured by the second timer is greater than or equal to apredetermined time threshold E2. If the time measured by the secondtimer is greater than or equal to the predetermined time threshold E2,step S310 is proceeded to. At step S310 the work phase determinationpart 62 finishes measurements of the second timer in the same manner asdescribed above, and at step S301 sets the boom pressure lower flag toOFF. If time measured by the second timer is not greater than or equalto the predetermined time threshold E2, the processing returns to stepS309.

Note that at step S311 if boom pressure is not less than the second boompressure determination value, step S313 is implemented. At step S313 thework phase determination part 62 resets the second timer and returns tostep S309.

FIG. 14 is a flowchart showing the processes for determining whether ornot the working implement raising hydraulic stall flag is ON. As shownin FIG. 14, at step S401 the working implement raising hydraulic stalldetermination part 63 sets the working implement raising hydraulic stallflag to OFF.

At step S402, the working implement raising hydraulic stalldetermination part 63 determines whether or not discharge pressure ofthe second hydraulic pump 2 (hereinafter “second pump pressure”) is lessthan boom pressure. The working implement raising hydraulic stalldetermination part 63 determines whether or not the second pump pressureis less than boom pressure based on detection signals from the boompressure sensor 22 and the discharge pressure sensor 39. Second pumppressure being below boom pressure means drive power from the secondhydraulic pump 2 is overpowered by the load applied to the lift cylinder19.

At step S403 the working implement raising hydraulic stall determinationpart 63 determines whether or not second pump pressure is less than orequal to a predetermined pump pressure threshold H1. The workingimplement raising hydraulic stall determination part 63 determineswhether or not the second pump pressure is less than or equal to pumppressure threshold H1 based on detection signals from the dischargepressure sensor 39. Pump pressure threshold H1 is equivalent to themagnitude of tractive force sufficient for performance of a scooping inoperation.

At step S404 the working implement raising hydraulic stall determinationpart 63 determines whether or not boom raise PPC pressure is greaterthan or equal to a first PPC pressure threshold G1. Boom raise PPCpressure is pilot pressure that arises due to a boom raise operationfrom the working implement operating member 23. That is to say, boomraise PPC pressure corresponds to the operation amount of the workingimplement operating member 23 performed in order to move the boom 53 inthe direction to raise it upwards. Boom raise PPC pressure is detectedthrough the PPC pressure sensor 21 described above. The first PPCpressure threshold G1 is a value sufficient to enable it to be deemedthat an operator is performing a boom raise operation.

At step S405 the working implement raising hydraulic stall determinationpart 63 determines whether the FNR recognition value is F or R.

When all of the conditions from step S402 to step S405 are satisfied,step S406 is implemented. At step S406 the working implement raisinghydraulic stall determination part 63 determines whether or not theworking implement raising hydraulic stall flag is OFF. If the workingimplement raising hydraulic stall flag is OFF, step S407 is implemented.

At step S407, the working implement raising hydraulic stalldetermination part 63 sets the working implement raising hydraulic stallflag to ON. Further, at step S408, the working implement raisinghydraulic stall determination part 63 commences measuring the amount ofchange in boom angle. That is to say, the working implement raisinghydraulic stall determination part 63 measures the amount of change inboom angle from the point in time at which the working implement raisinghydraulic stall flag is set to ON. Step S404 is then returned to.

If the conditions of step S404 among the above described conditions isnot satisfied, step S409 is proceeded to. At step S409 the workingimplement raising hydraulic stall determination part 63 determineswhether or not boom raise PPC pressure is less than or equal to thesecond PPC pressure threshold G2. The second PPC pressure threshold G2is a value that shows an amount of operation of the working implementoperating member 23 sufficient to enable it to be deemed that theoperator is not performing a boom raise operation. At step S409 if theboom raise PPC pressure is less than or equal to the second PPC pressurethreshold G2, step S410 is proceeded to.

At step S410, the working implement raising hydraulic stalldetermination part 63 sets the working implement raising hydraulic stallflag to OFF. Accordingly when the working implement raising hydraulicstall flag is ON, at step S410 the working implement raising hydraulicstall determination part 63 returns the working implement raisinghydraulic stall flag to OFF. If the working implement raising hydraulicstall flag is OFF, at step S410 the working implement raising hydraulicstall determination part 63 maintains that flag in OFF. That is to say,the working implement raising hydraulic stall determination part 63determines that the working implement 52 is not in the raising hydraulicstall condition when the boom raising operation has ceased to beperformed by the working implement operating member 23. Further, at stepS411 the working implement raising hydraulic stall determination part 63resets the measurement of amount of change of boom angle. That is tosay, the working implement raising hydraulic stall determination part 63measures the amount of change in boom angle for the duration in whichworking implement raising hydraulic stall is ON.

At step S409 if boom raise PPC pressure is not less than or equal tosecond PPC pressure threshold G2, step S405 is proceeded to. That is tosay if boom raise PPC pressure is a value between first PPC pressurethreshold G1 and second PPC pressure threshold G2, no change is made tothe working implement raising hydraulic stall flag and the currentcondition is maintained.

If the conditions of step S405 are not satisfied, the working implementraising hydraulic stall determination part 63 at step S410, sets theworking implement raising hydraulic stall flag to OFF. That is to say,the working implement raising hydraulic stall determination part 63determines that the working implement 52 is not in the raising hydraulicstall condition if the forward/reverse control member 14 is in theneutral position.

If the conditions at step S406 are not satisfied, step S412 isperformed. At step S412 the working implement raising hydraulic stalldetermination part 63 determines whether or not the amount of change inboom angle is greater than or equal to an angle change amount thresholdB3. The angle change amount threshold B3 is an amount of change in boomangle sufficient to enable it to be considered that movement of the boom53 has not stopped. It is preferable for the angle change amountthreshold B3 to be less than or equal to 3°. The angle change amountthreshold B3 can for example be 1°. When the amount of change in boomangle is greater than or equal to the angle change amount threshold B3,the working implement raising hydraulic stall determination part 63 setsthe working implement raising hydraulic stall flag to OFF.

If the conditions at step S402 and step S403 are satisfied, thedeterminations from step S402 onward are repeated. If the condition atstep S412 is not satisfied, step S404 is returned to.

In the wheel loader 50 according to this embodiment, if the abovedescribed determination conditions are satisfied during the tractiveforce control, the control level of tractive force is lowered from thefirst level to the second level. In this way maximum tractive force ismade to reduce. If the determination conditions are satisfied, it meansthat in a condition in which during excavation work there is sufficienttractive force present, when the operator attempts to raise the bucket54 the bucket 54 does not rise. The wheel loader 50 according to thisaspect enables the working implement 52 to recover from this raisinghydraulic stall condition by automatically lowering maximum tractiveforce when such a condition occurs. Further, as it is not necessary forthe operator to perform any operation in order to change the level ofmaximum tractive force, improved operability can be realized.

The above described determination conditions operate such that whenengine rotation speed is smaller than maximum torque rotation speed,maximum tractive force is not lowered. In this way, when maximumtractive force is made to reduce, occurrence of the phenomenon in whichengine rotation speed suddenly decreases is prevented.

With the above described determination conditions, the working implementraising hydraulic stall determination part 63 can determine if the boom53 is in the lowered condition by determining whether or not the boomangle is less than or equal to a predetermined boom angle threshold.This enables accurate determination of the condition in which the boom53 in the lowered condition is unable to be raised.

An operator, by operating the settings operation device 24, can changethe magnitude of maximum tractive force of the first level. If thedetermination conditions are satisfied, the tractive force control part61 lowers maximum tractive force to a value smaller than maximumtractive force of the first level. In this way, the operator is able tofinely set the required maximum tractive force in conformance with workconditions.

If the determination conditions cease to be satisfied during thetractive force control, the tractive force control part 61 returns thecontrol level of tractive force to the first level. This enables theappropriate maximum tractive force in conformance with work conditionsto be obtained.

The tractive force control part 61, when returning the tractive forcecontrol level from the second level to the first level, changes pumpdisplacement more slowly than when lowering the tractive force controllevel from the first level to the second level. This enables suddenincrease of tractive force to be prevented, thereby enabling preventionof the occurrence of slips and providing improved operability. Further,the tractive force control part 61, when lowering the control level oftractive force from the first level to the second level, changes pumpdisplacement more quickly than when returning the level of the tractiveforce control from the second level to the first level. Thus, in thecase of the wheel loader 50 of this embodiment, when the workingimplement 52 has fallen into a condition of raising hydraulic stall thecondition can be swiftly rectified.

Although the invention has been described above by reference to anembodiment thereof, the invention is not limited to the embodimentdescribed above. It is therefore understood that numerous modificationsand variations can be devised without departing from the scope of theinvention.

The above embodiment has been described with reference to an example ofa wheel loader 50 mounting an HST system of one pump one motor,including a single hydraulic pump and a traveling hydraulic motor 10.The invention is however, not limited to the embodiment described above.For example, it is also suitable for the present invention to be appliedto a wheel loader mounting an HST system of one pump two motors,including a single first hydraulic pump and two traveling hydraulicmotors.

In the case of the above described embodiment of the present invention,the settings operation device 24 is able to change maximum tractiveforce of the first level through three stages. However, it is alsosuitable if the settings operation device 24 is capable of changingmaximum tractive force of the first level in a plurality of stages otherthan three stages. Alternatively, it is suitable for the settingsoperation device 24 to be capable of consecutively freely changing themagnitude of maximum tractive force of the first level. Again, it isalso suitable for the settings operation device 24 to be omitted, thatis to say it is suitable if the maximum tractive force of the firstlevel is unable to be changed.

The determination conditions are not limited to those described aboveand it is suitable for others to be added. Again, it is also suitablefor the above described determination conditions to be changed in part.

In the case of the above described embodiment of the present invention,the tractive force control part 61 lowers maximum tractive force bychanging the upper limit displacement of motor displacement, however itis also suitable for maximum tractive force to be lowered by othermethods. It is suitable for example for the tractive force control part61 to lower maximum tractive force by controlling drive circuitpressure. Drive circuit pressure can for example be controlled bycontrolling displacement of the first hydraulic pump 4.

In the case of the above described embodiment of the present invention,tractive force ratio information is set such that the tractive forceratio increases in conformance with the increase in acceleratoroperation amount, it is also suitable however for tractive force ratioinformation to be set such that tractive force ratio is constant,regardless of accelerator operation amount.

The illustrated embodiment provides a wheel loader and method forcontrolling a wheel loader that enables a working implement to recoverfrom a condition of raising hydraulic stall during excavation work andprovides improved operability.

What is claimed is:
 1. A wheel loader comprising: a working implementincluding a boom, a bucket and a lift cylinder that raises the bucket bymoving the boom; an engine; a first hydraulic pump driven by the engine;a traveling hydraulic motor driven by hydraulic fluid discharged fromthe first hydraulic pump; a second hydraulic pump driven by the engineto discharge hydraulic fluid that drives the lift cylinder; a workingimplement operating member configured to operate the working implement;a drive circuit pressure detection part configured to detect a drivecircuit pressure that is a pressure of the hydraulic fluid for drivingthe traveling hydraulic motor; a raising hydraulic stall determinationpart configured to determine whether or not the working implement is ina raising hydraulic stall condition, in which the bucket does not riseregardless of operation of the working implement operating member; adrive circuit pressure determination part configured to determinewhether or not the drive circuit pressure is greater than or equal to apredetermined hydraulic pressure threshold; and a tractive force controlpart configured to perform a tractive force control to reduce a maximumtractive force, wherein the tractive force control part is furtherconfigured to reduce the maximum tractive force when determinationconditions including, while the working implement is performingexcavation operation, that the working implement is in the raisinghydraulic stall condition and that the drive circuit pressure is greaterthan or equal to the predetermined hydraulic pressure threshold aresatisfied during the tractive force control.
 2. The wheel loaderaccording to claim 1, further comprising: an engine rotation speeddetection part configured to detect engine rotation speed; and an enginerotation speed determination part configured to determine whether or notthe engine rotation speed is greater than or equal to a predeterminedrotation speed threshold, wherein the determination conditions furtherinclude that the engine rotation speed is greater than or equal to thepredetermined engine rotation speed threshold.
 3. The wheel loaderaccording to claim 1, further comprising a boom angle determination partconfigured to determine whether or not a boom angle that is an angle ofthe boom in relation to a horizontal direction, is less than or equal toa predetermined angle threshold, wherein the determination conditionsfurther include that the boom angle is less than or equal to thepredetermined angle threshold.
 4. The wheel loader according to claim 1,wherein for the tractive force control, the tractive force control partis configured to set a control level of tractive force, which isindicative of a reduction amount of the maximum tractive force, at afirst level, and the tractive force control part is configured to changethe control level of the tractive force to a second level, the maximumtractive force of which is less than the maximum tractive force of thefirst level when the determination conditions are satisfied during thetractive force control.
 5. The wheel loader according to claim 4,further comprising a tractive force level changing part configured tochange magnitude of the maximum tractive force of the first level. 6.The wheel loader according to claim 4, wherein the tractive force levelchanging part is configured to return the tractive force control levelto the first level when the determination conditions cease beingsatisfied during the tractive force control.
 7. The wheel loaderaccording to claim 6, wherein the tractive force level changing part isconfigured to change tractive force when returning the tractive forcecontrol level to the first level, more slowly than when changing thetractive force control level to the second level.
 8. The wheel loaderaccording to claim 1, wherein the tractive force control part isconfigured to determine whether or not the working implement isperforming the excavation operation based on a condition of travel ofthe wheel loader and an operating state of the working implement.
 9. Thewheel loader according to claim 1, further comprising a boom pressuredetection part configured to detect a pressure of hydraulic fluidsupplied to the lift cylinder, wherein the raising hydraulic stalldetermination part is configured to determine whether or not the workingimplement is in the raising hydraulic stall condition based on anoperation amount of the working implement operating member and thepressure of hydraulic fluid supplied to the lift cylinder.
 10. The wheelloader according to claim 1, further comprising a forward/reverse travelcontrol member configured to change a direction of travel of the wheelloader by switching between a forward travel position, a reverse travelposition and a neutral position, wherein the raising hydraulic stalldetermination part is configured to determine that the working implementis not in the raising hydraulic stall condition when the forward/reversetravel control member is in the neutral position.
 11. The wheel loaderaccording to claim 2, wherein the predetermined rotation speed thresholdis, for an output torque line that shows a relationship between theengine rotation speed and an upper limit of engine torque the engine canoutput at each engine rotation speed, the engine rotation speed when theupper limit of the engine torque reaches the maximum.
 12. The wheelloader according claim 1, wherein the tractive force control part isconfigured to control a displacement of the traveling hydraulic motor bycontrolling a tilting angle of the traveling hydraulic motor and toperform control of the maximum tractive force by controlling an upperlimit displacement of the displacement of the traveling hydraulic motor.13. The wheel loader according to claim 1, wherein the tractive forcecontrol part is configured not to reduce the maximum tractive force whenthe working implement is not performing the excavation operation. 14.The wheel loader according to claim 1, wherein the tractive forcecontrol part is configured not to reduce the maximum tractive force whenthe working implement is not in the raising hydraulic stall condition.15. The wheel loader according to claim 1, wherein the tractive forcecontrol part is configured not to reduce the maximum tractive force whenthe drive circuit pressure is not greater than or equal to thepredetermined hydraulic pressure threshold.
 16. The wheel loaderaccording to claim 2, wherein the tractive force control part isconfigured not to reduce the maximum tractive force when the enginerotation speed is not greater than or equal to the predeterminedrotation speed threshold.
 17. The wheel loader according to claim 3,wherein the tractive force control part is configured not to increasethe maximum tractive force when the boom angle is less than or equal tothe predetermined angle threshold.
 18. A method for controlling a wheelloader including an engine and a working implement including a boom, abucket and a lift cylinder that raises the bucket by moving the boom,the method comprising: determining whether or not a raising hydraulicstall condition is occurring, in which the bucket does not rise eventhough hydraulic fluid discharged from a second hydraulic pump, which isdriven by the engine, is supplied to the lift cylinder; determiningwhether or not a drive circuit pressure of a drive circuit, throughwhich hydraulic fluid discharged from a first hydraulic pump driven bythe engine is supplied to a traveling hydraulic motor, is greater thanor equal to a predetermined hydraulic pressure threshold; and during atractive force control for reducing a maximum tractive force isperformed, further reducing a maximum tractive force when determinationconditions including, while the working implement is performingexcavation operation, that the raising hydraulic stall condition isoccurring and that the drive circuit pressure is greater than or equalto the predetermined hydraulic pressure threshold are satisfied.